Hematophagous (blood-feeding) insects and their symbionts face a unique challenge - accessing the rich supply of iron in the blood meal, while avoiding the toxicity associated with high levels of free iron and heme from the digested blood meal in the insect gut. Meeting this challenge is important for survival, and in the case of vector of disease, may play a role in disease transmission. Sodalis glossinidius is a secondary symbiont of one such hematophagous insect - the tsetse fly (Genus: Glossina). The tsetse fly, which feeds on vertebrate blood, is the vector of protozoan African trypanosomes (Trypanosoma sp), the causative agents of human and animal African trypanosomiases. The central hypothesis of this proposed work is that Sodalis' genome encodes a suite of heme-induced proteins that enable the bacterium to survive heme toxicity. Thus, the scientific goals of this project are to identify and characterize Sodalis heme tolerance genes, which encode proteins that protect the bacterium against toxicity from blood meal derived heme. Sodalis is one of the few insect symbionts that can be grown in culture and genetically manipulated, which provides a relatively unique opportunity to do functional genetic studies of potential heme tolerance genes. In specific aim 1, Sodalis genes that are induced by exposure to heme will be identified through global transcriptome analysis. The rationale behind this aim, as related to the central hypothesis, is that heme will likely induce the expression of many genes that encode proteins important for mitigating heme toxicity. In specific aim 2, mutations will be constructed i the heme-induced genes to deduce function of these genes. The phenotypes of the mutants with respect to heme tolerance and heme-mediated biomolecule damage (specific aim 2) and with respect to survival/proliferation in tsetse flies (specific aim 3) will be examined. Studying he tsetse fly-Sodalis symbiosis will have significance to public health. The presence of Sodalis in the tsetse fly may increase tsetse-mediated transmission of the trypanosome, and recent research suggests that paratransgenesis may be a useful control strategy for African trypanosomiasis. For these reasons, a more complete understanding of the nature of the Sodalis-tsetse symbiosis will yield information to help control tsetse-borne disease.